The invention concerns an implement used in trawler fishing, commonly known as trawler doors or otter boards, which are utilized for spreading the net in the water behind a towing fishing vessel. The invention relates especially to the kind of trawler doors known as the cambered Vee-type, in which two cambered platelike parts are joined along their longest edge, forming an obtuse angle between them.
The use of trawler doors is illustrated in U.S. Pat. No. 4,045,901. Cambered Vee-type trawler doors are disclosed in Great Britain Pat. No. 855,746 where the camber may be described as an aerofoil section when looking at the door from the top. In such a aerofoil section the trailing part has a larger curving radius or may be almost straight compared with the leading part which has a rather small radius of curvature.
In use for demersal fishing on or close to a rocky bottom, as in some waters off the U.S. coast, trawler doors suffer rough treatment when they hit obstacles as rocks or cliffs. Also, the doors are subject to metal fatigue since the pulling force from the fishing vessel during fishing is transferred through the warps to the doors and further on to the trawl net, which force varies according to the circumstances when fishing, hauling up the catch, and when the doors are out of use. As the chains connecting the net to the door are fastened to the platelike parts of the door, the door is subjected to considerable bending moments, which tend to bend the door along a line parallel to the longitudinal axis of the door.
In order to counter these loads and to prevent an early breakdown of thedoors, stiffning members or struts with a T-shaped cross-section have been provided stretching mainly vertically from the central rib to the edge of the door. The T-shaped members are disadvantageous, however, in respect to the hydrodynamic resistance produced by the part of the T-shaped members extending transversely to the waterflow past the members while fishing with the door.